Semiconductor manufacturers are continually trying to achieve shallow junctions to control the threshold voltage (V.sub.T) and transconductance. However, a problem in forming such shallow junctions is the ability to etch a dielectric layer which overlies the junction region without damaging the junction region. In etching an opening in the dielectric layer, the junction region is exposed, and can be physically or chemically attacked. If the junction is too shallow, the dielectric etch will damage the junction region, and in the worst case completely punch through the junction region rendering the device inoperable due to substrate electrical short-circuiting.
One known solution to help alleviate the problems of junction breakthrough is to use a plasma enhanced nitride (PEN) as an etch stop layer when forming contacts and local interconnects. This blanket etch stop layer is deposited across the entire semiconductor substrate. This PEN layer is deposited prior to the first interlayer dielectric material. In etching local interconnect or contact openings in the interlayer dielectric, the PEN layer serves as an etch stop over the junctions. Contacts are formed to both the substrate (source and drain) and the gate electrode regions. The bottom-most layer of the contact structure is usually silicon substrate or field/trench isolation regions. The top most contacted regions are the gate electrodes. The presence of the PEN etch stop layer prevents the top polysilicon member from being exposed to the interlayer dielectric etch while the etch is still in progress to remove the interlayer dielectric over the junction/substrate regions. However, the PEN etch stop layer above the junctions in polysilicon regions nonetheless has to be removed to make electrical contacts to the gate and substrate members. In removing the PEN etch stop layer in these areas, there is still the risk that the etch of the PEN layer will result in junction punch-through.
Silicide regions are often formed over the junction regions and polysilicon members to improve contact resistance. In the past, such silicide regions have been somewhat effective in preventing junction punch-through. However, as junctions become more shallow, the silicide regions become thinner, and the silicide regions are proving less than effective in preventing junction punch-through.
Accordingly, there is a need in the semiconductor industry for a method for forming shallow junctions in semiconductor devices.